N-(phosphonomethyl)glycine and its salts are conveniently applied as a post-emergent herbicide in aqueous formulations. It is a highly effective and commercially important broad-spectrum herbicide useful in killing or controlling the growth of a wide variety of plants, including germinating seeds, emerging seedlings, maturing and established woody and herbaceous vegetation, and aquatic plants.
One of the more widely accepted methods of making N-(phosphonomethyl)glycine compounds comprises liquid phase oxidative cleavage of a carboxymethyl substituent from an N-(phosphonomethyl)iminodiacetic acid substrate using an oxygen-containing gas in the presence of a heterogenous oxidation catalyst. For example, N-(phosphonomethyl)glycine may be prepared by the liquid phase oxidative cleavage of N-(phosphonomethyl)iminodiacetic acid with oxygen in accordance with the following reaction: Other by-products also may form, such as formic acid, which is formed by the oxidation of the formaldehyde by-product, and aminomethylphosphonic acid (“AMPA”), which is formed by the oxidation of N-(phosphonomethyl)glycine. The preference for heterogenous catalysis stems, at least in part, from the ease with which a particulate heterogeneous catalyst can normally be separated from the reaction product mixture for reuse following the oxidation. The literature is replete with examples of heterogeneous catalysts. See generally, Franz, et al., Glyphosate: A Unique Global Herbicide (ACS Monograph 189, 1997) at pp. 233-62 (and references cited therein); Franz, U.S. Pat. No. 3,950,402; Hershman, U.S. Pat. No. 3,969,398; Felthouse, U.S. Pat. No. 4,582,650; Chou, U.S. Pat. Nos. 4,624,937 and 4,696,772; Ramon et al., U.S. Pat. No. 5,179,228; and Ebner et al., U.S. Pat. No. 6,417,133.
High concentrations of formaldehyde in the reaction product mixture resulting from oxidative cleavage of an N-(phosphonomethyl)iminodiacetic acid substrate is undesirable. The formaldehyde by-product is undesirable because it reacts with N-(phosphonomethyl)glycine to produce unwanted by-products, mainly N-methyl-N-(phosphonomethyl)glycine (“NMG”), which reduce the N-(phosphonomethyl)glycine yield. In addition, the formaldehyde by-product itself is undesirable because of its potential toxicity. See Smith, U.S. Pat. No. 5,606,107.
Franz, U.S. Pat. No. 3,950,402, discloses oxidizing the formaldehyde by-product to carbon dioxide and water simultaneous with the oxidative cleavage of the N-(phosphonomethyl)iminodiacetic acid substrate by using a heterogenous oxidation catalyst comprising a noble metal deposited on a carbon support. The noble metal on carbon oxidation catalyst may be referred to as “bifunctional” since the carbon component provides the primary adsorption site for the oxidation of the N-(phosphonomethyl)iminodiacetic acid substrate to form the N-(phosphonomethyl)glycine product and formaldehyde, while the noble metal component provides the primary adsorption site for the oxidation of formaldehyde and formic acid to form carbon dioxide and water, thus giving the following reaction: The noble metal component may also tend to reduce the rate of deactivation of the catalyst (i.e., prolong the useful life of the catalyst). However, under typical conditions of the oxidation reaction, some of the noble metal in the catalyst used by Franz is oxidized into a more soluble form and both the N-(phosphonomethyl)iminodiacetic acid and N-(phosphonomethyl)glycine act as ligands which solubilize the noble metal. Thus, even though the process disclosed by Franz produces an acceptable yield and purity of N-(phosphonomethyl)glycine, high losses of the costly noble metal by dissolution into the aqueous reaction solution (i.e., “leaching”) undermine the economic feasibility of the process.
Ramon et al., U.S. Pat. No. 5,179,228, disclose a process for the preparation of N-(phosphonomethyl)glycine by oxidation of N-(phosphonomethyl)iminodiacetic acid using an oxygen-containing gas in the presence of a noble metal on activated carbon catalyst. Recognizing the problem of leaching attendant the use of a noble metal on carbon catalyst in the oxidation of an N-(phosphonomethyl)iminodiacetic acid substrate (noble metal losses as great as 30% are reported), Ramon et al. propose flushing the reaction mixture with nitrogen gas under pressure after the oxidation reaction is complete. According to Ramon et al., nitrogen flushing causes redeposition of solubilized noble metal onto the surface of the carbon support and reduces the noble metal loss to less than 1%. However, the nitrogen flushing treatment taught by Ramon et al., which may last up to an hour, is conducted on the entire reaction mixture containing the N-(phosphonomethyl)glycine product in contact with the oxidation catalyst. Such prolonged contact after the oxidation reaction is complete may lead to considerable product losses through methylation as the N-(phosphonomethyl)glycine reacts with formaldehyde to produce N-methyl-N-(phosphonomethyl)glycine and other undesirable by-products. In addition, repeated dissolution and redeposition of the noble metal can lead to loss of noble metal surface area through sintering (i.e., the formation of undesirably thick layers or agglomerated clumps) which, in turn, decreases the activity of the catalyst.
More recently, attention has focused on developing bifunctional noble metal on carbon oxidation catalysts which resist noble metal leaching (i.e., exhibit improved compositional stability) and provide increased activity and/or selectivity, particularly with respect to oxidation of formaldehyde into carbon dioxide and water (i.e., increased formaldehyde activity). Ebner et al., U.S. Pat. No. 6,417,133, disclose so-called “deeply reduced” noble metal on carbon catalysts for use in the oxidative cleavage of an N-(phosphonomethyl)iminodiacetic acid substrate and oxidation of other oxidizable reagents and methods for their preparation. Such deeply reduced catalysts exhibit remarkable resistance to noble metal leaching in aqueous, acidic oxidation reaction media. As a result, the catalyst disclosed by Ebner at al. provides for substantially quantitative oxidation of N-(phosphonomethyl)iminodiacetic acid substrates to N-(phosphonomethyl)glycine products while maintaining effective oxidation of the formaldehyde and formic acid by-products of the reaction for a prolonged period and/or over numerous oxidation cycles.
Although the teachings of Ebner et al. are significant and make economically practical the otherwise unavailable advantages provided by noble metal on carbon catalysts in the preparation of N-(phosphonomethyl)glycine products by oxidative cleavage of N-(phosphonomethyl)iminodiacetic acid substrates, a need persists for improvements which might further reduce noble metal losses, provide increased catalyst stability and activity and/or selectivity, particularly in the oxidation of formaldehyde and other N-(phosphonomethyl)iminodiacetic acid substrate oxidation by-products, and generally extend the useful life of such catalysts.